Clean up Cypher parser helpers - #1515

Closes #1510.

Summary

• remove stale Cypher parser transformer hooks that no longer map to grammar rules
• DRY MATCH and OPTIONAL MATCH AST construction through one private helper
• add parser coverage for optional bound and optional shortestPath alias metadata
• update CHANGELOG.md

Validation

• python3 -m pytest -q graphistry/tests/compute/gfql/cypher/test_parser.py -k "optional_bound_pattern or optional_shortest_path"
• python3 -m pytest -q graphistry/tests/compute/gfql/cypher/test_parser.py
• python3 -m pytest -q graphistry/tests/compute/gfql/cypher/test_lowering.py -k "(shortestPath or bound_variable_length_relationship_pattern_alias or optional_match) and not cudf"
• ./bin/ruff.sh graphistry/compute/gfql/cypher/parser.py graphistry/tests/compute/gfql/cypher/test_parser.py
• ./bin/typecheck.sh graphistry/compute/gfql/cypher/parser.py
• git diff --check origin/master...HEAD
• review skill wave 3 refresh after rebase: converged, no findings
• ssh dgx-spark RAPIDS 25.02 gfql: parser suite + explicit cuDF optional-match sentinels, 148 passed
• ssh dgx-spark RAPIDS 26.02 gfql: parser suite + explicit cuDF optional-match sentinels, 148 passed

Rebase Notes

• Rebased over origin/master f183130 after Clean up AST helpers #1513/refactor(gfql): remove unused cypher lowering delegate #1516; conflict was limited to CHANGELOG.md and kept both current master GFQL cleanup: compute ast.py dead-code and node helper audit #1512 and this GFQL cleanup: parser.py dead-code and grammar helper audit #1510 internal entries.
• Rebased again over origin/master 6501632 after Remove residual Cypher chain fallback #1508 landed; no conflicts, focused local validation rerun green.
• Current net diff: 3 files, 26 insertions, 28 deletions.

Notes

• Local cuDF-selected lowering run hit cudaErrorNoDevice on this host; the same explicit cuDF sentinels passed on dgx-spark for RAPIDS 25.02 and 26.02.
• No DGX rerun was needed after the rebase because the conflict was changelog-only and the code diff remains parser-only.